This invention relates to a pneumatic tire having a rubber insert in its sidewall axially inward of at least one carcass ply. Such insert may be, for example, an apex in extending radially outward from a bead core of a tire into its sidewall. Alternatively, the insert may be positioned higher in the sidewall portion of the tire and away from the bead core. Such insert is a rubber composition containing a dispersion of an ultra high molecular weight polyethylene and a dispersion of a starch composite.
It is sometimes desired to provide a tire having a sidewall of a relatively high stiffness to enhance a tire""s handling, for example its cornering stability, when mounted on a rigid rim as a part of a wheel of a vehicle. For example, see U.S. Pat. Nos. 4,024,901 and 4,067,373.
Often, a resin is added to an apex portion of a tire sidewall to make it stiffer. However, addition of a resin for such purpose, while increasing hardness for the rubber composition, may also adversely affect the rubber composition""s other desirable properties such as, for example, its rebound and permanent set properties.
Another method of increasing hardness for a tire apex is to increase its reinforcing filler content (e.g.: its carbon black content). However, increasing the carbon black content for such rubber composition, while increasing its hardness, may adversely affect physical properties such as, for example, its hysteresis and, therefore, its heat build-up property and also may disadvantageously increase its stiffness softening with strain history property.
The stiffness softening property relates to a comparative difference (e.g.: a reduction) between a first force (MPa) needed for application of about a one percent shear strain at 100xc2x0 C. to the rubber sample and a second force (MPa) needed for the same shear strain; wherein an intermediate and significantly greater shear strain (e.g.: about a 50 percent shear strain) is applied to the sample with an accompanying relaxation of the sample to its original size after each of said first and intermediate shear strain applications. If there is only a minimal, if any, decrease in the second force, then it may be said that there is little, if any, stiffness softening of the cured rubber sample.
In practice, a low stiffness softening of the rubber composition for the insert is desired.
In the description of this invention, a stiffness of a cured rubber composition is a characteristic similar to modulus in a sense that it is measured in terms of MPa and relates to a force required to obtain a prescribed strain, or elongation, of a rubber composition.
In practice, it is desired that a rubber composition for the insert have a relatively high stiffness, or modulus, at an elongation of 100 percent. While of a somewhat lesser significance, it is also desired that it also have a relatively high modulus at a larger elongation of 300 percent.
For this invention, variation in methodology of increasing stiffness for a tire sidewall insert is desired.
In particular, it is desired herein to provide a tire with a sidewall having an insert which has a relatively high hardness represented as having a Shore D hardness (23xc2x0 C.) in a range of about 35 to about 50, yet also having a Hot Rebound value at 100xc2x0 C. of about 65 to about 80 in order to reduce heat generation in the rubber composition under working conditions as a tire sidewall insert.
In practice, for the relatively hard rubber compositions for the insert of this invention, the hardness values may be desired to be recited in terms of a Shore D hardness rather than a Shore A hardness.
Historically, a tire is a composite of numerous components each serving a specific and unique function and cooperatively functioning to produce a desired tire performance. In one aspect, a tire carcass may contain one or more reinforcing carcass plies. A carcass ply is typically a continuous layer of rubber-coated parallel cords which extend from bead to bead and functions as a reinforcing element of the tire. The plies are usually turned up around the bead, thereby locking the bead into the carcass. Such carcass reinforcing plies are well known to those having skill in such art.
In one aspect of the invention, a tire sidewall insert may be a tire apex. The term xe2x80x9capexxe2x80x9d as used herein refers to a rubber wedge located primarily in the lower sidewall region extending radially outward from the tire""s bead core into the sidewall of the tire, usually between a carcass ply and its turn-up portion. An apex portion of a tire is well known to those skilled in such art.